Fate And Transport Of Phosphorus, Colloids, And Biochar In Soils

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Zhang, Wei

Abstract

Fate and transport of phosphorus (P) and colloidal particles in soils constitutes two major areas of inquires in contaminant hydrology. The P transport in the northeastern U.S. depends largely on P sorption of soils in variable source areas (VSAs) and land treatment systems (e.g., vegetative treatment areas [VTAs]) that receive large P applications. In this study P sorption of soils from VTAs receiving dairy farm wastewaters in New York was studied using batch P sorption experiments. A modified P sorption model that uses only Langmuir sorption isotherm was compared with a conventional model that needs both linear and Langmuir isotherms for sixteen VTA soil samples of Langford, Volusia, and Mardin channery silt loam soils. The two methods agreed well in describing P sorption, thus proving the modified model a valid tool for P sorption study. Then, the modified model was employed to study the effect of soil redox change on P sorption of an organic-rich Langford silt loam. The findings suggest that with soil redox fluctuation, invoked by alternating saturated and unsaturated soil moisture regimes, soil P sorption was enhanced by formation of freshly precipitated amorphous iron hydroxides. Colloid transport was investigated using a model colloid (carboxylated polystyrene microsphere) and a non-ideal colloid (biochar particles) in sand columns. For the model colloid, greater input concentrations lead to increased colloid retention at neutral pH in unsaturated sand, and this concentration effect was enhanced by ionic strength. This has a direct consequence for predicting the transport of colloids where the concentration decreases with depth and thus become more mobile with depth. It is expected that large quantities of biochar might be produced in the future for carbon sequestration. Therefore, the transport of biochar particles was investigated under three pH and two ionic strength levels in saturated and unsaturated sand. Biochar particles exhibited greater mobility under higher pH and lower ionic strength, and more biochar was transported by saturated flow. The biochar particles larger than 5.4% of median grain diameter were filtered out of suspension during passage through the media, whereas the retention of smaller particles was clearly dependent on solution chemistry.